1. Field of the Invention
The present invention relates to a photovoltaic cell comprising a non-monocrystalline semiconductor.
2. Related Background Art
A photovoltaic cell comprising a non-monocrystalline semiconductor generally has a pin junction. Generally such a photovoltaic cell has a p-layer as a window layer, i.e., on the light incident side.
According to S. Guha, J. Yang, P. Nath and M. Hack, "Enhancement of open circuit voltage in high efficiency amorphous silicon alloy solar cells", Appl. Phys. Lett., 49 (1986) 218, by using the physical properties of microcrystalline silicon (.mu.c-Si:H) such as a high conductivity and a small absorption coefficient in the short wavelength region, .mu.c-Si:H is used as the p-layer on the light incident side to reduce the short circuit current (J.sub.sc). In addition, since .mu.c-Si:H has a wider gap than that of a-Si:H, it has a high impurity doping efficiency, and the internal electric field in a photovoltaic cell using .mu.c-Si:H increases. As a result, this improves the open circuit voltage (V.sub.oc) and the photoelectric conversion efficiency (.eta.).
When the p-layer on the light incident side consisting of .mu.c-Si:H is forward-biased, however, injection of holes from the upper electrode is promoted, because the p-layer has a low activation energy, thereby increasing the dark current. Therefore, it may occur that the open circuit voltage V.sub.oc once raised by the use of .mu.c-Si:H in the p-layer drops again. The present inventor speculated that a higher open circuit voltage V.sub.oc may be obtained when the above problem is solved, thereby achieving the present invention.